1. Field of the Invention
The invention relates to the calibration of a color printer. More particularly, the invention relates to a calibration apparatus and method for a color printer, where the calibration is performed in such a way that prior training and expensive equipment are not required.
2. Description of Prior Art
Many printing technologies, including electrophotographic and ink jet technology, are complex and, due to many different physical and environmental factors, may drift in their color response, i.e. the amount of color toner or ink that is printed on a paper. That is, a particular combination of input colorants to the printer, cyan, magenta, yellow, and black, or (c,m,y k), results in a different color response when printed to one printer at different times or when printed to different printers of the same model. Preferably, the color response should be steady through time and consistent between different devices. Prior art systems attempt to compensate for the difference in color behavior of the measured response of the printer and the goal response by various calibrating techniques.
Users currently calibrate a printer or copier using a densitometer or a scanner. For purposes of the invention described herein, calibration means adjustments to single toner or ink values, through one-dimensional lookup tables, to provide for steady, predictable color printing. This document describes the goals, techniques and a proposed development plan for a calibration system that involves neither a densitometer nor a scanner but that uses visual comparisons by an untrained user.
For a general description of the process of generating color documents, refer to R. J. Rolleston, Color Printer Calibration Method For Accurately Rendering Selected Colors, U.S. Pat. No. 5,689,350, Nov. 18, 1997 and R. J. Rolleston et al., Color Printer Calibration Test Pattern, U.S. Pat. No. 5,416,613, May 16, 1995, in which the word calibration in the titles preceding do not mean calibration in the sense of the invention. In one approach, the generation of color documents can be thought of as a two step process: first, the generation of the image by means of scanning an original document with a color image input terminal or scanner or, creating a color image on a workstation; and secondly, printing of the image with a color printer in accordance with the colors defined by the scanner or computer generated image. Scanner output is commonly transformed to a color space of calibrated RGB (red-green-blue). In another approach, computer generated images can be defined initially in the color space of calibrated RGB. These colors are defined independently of any particular device and are referred to as device independent.
Most methods for calibration in the sense of the invention involve the use of possibly expensive instruments such as densitometers or scanners to measure the color response of the printer in comparison with a goal response, the color response and the goal response expressed in terms of density. In the methods an algorithm may be used to compensate for the difference in color behavior of the measured response of the printer and the goal response. Similarly, many methods for calibration require the use of highly trained operators. As described in the PCT Patent Application No. WO 92/01264, one such prior art system is employed by certain systems typically used in professional preprint shops. In this case, the user specifies a small number of aim points (usually three or four) on an image and controls the colors at those points. A major disadvantage of this method is that the process of specifying the aim points requires a highly trained operator.
A second prior art system also described in the PCT Patent Application No. WO 92/01264, is used by desktop computer scanning programs. Here, the user has control over a few parameters, such as brightness and contrast that control a mapping of the colors seen by the scanner. Again, a major disadvantage is this process requires a user with a significant amount of training.
A third prior art method described in PCT Patent Application No. WO 92/01264, is device dependent because it requires the scanner to measure the colors on the sheet. The resulting calibration is for the combination of scanner and printer, rather than for the printer itself. This method has some significant limitations. For example, this method does not allow a way to accurately display or print computer rendered images, i.e. images that do not originate as scanned images but rather are originally generated by the computer.
Instrument-based calibration is driven by measurements from instruments. Instrument based calibration leads to consistent results because the human factor is minimized. The time it takes to calibrate and the quality of the results are predictable. Although custom calibration goals need to be prepared by a skilled operator, a minimally trained operator can perform a very good calibration.
Human vision based calibration is also possible (see, for example, P. Engeldrum, W. Hilliard, Interactive Method and System for Color Characterization and Calibration of Display Device, U.S. Pat. No. 5,638,117 (Jun. 10, 1997)). Vision based calibration relies on the subjective perception by the human eye. Visual based calibration leads to more variable results than instrument based calibration. However, if designed well, the calibration process can be performed rapidly and can require much less data from the targets. If operator-oriented, the human is usually the less controllable element in the printing and evaluating workflow. The quality of calibration can vary from one operator to another.
P. Dundas, D. Temple, S. Zoltner, Printer Color and Gray Balance Adjustment System, U.S. Pat. No. 5,604,567 (Feb. 18, 1997), describes a gray balancing approach to copier or printer adjustment. Dundas discloses a technique that allows for printing of multi-colored pages to be used for visual comparisons. The disclosed technique works in conjunction with specific engine adjustments and controls (developer charge control, as an example) but does not generate digital transfer curves for printer control and therefore does not provide a uniform interface across printer models and technologies. In addition,
Dundas does not minimize the iterations needed and does not maximize the accuracy of the calibration in highlights and gray balance.
It would be wrong, however, to conclude that instrument based is always a better choice than visual based calibration:
Instruments cost money. The user may want good color quality, but may not be ready to purchase a densitometer, that is more expensive than many low end printers, preexisting reference targets. PA1 Instruments can deliver unreliable results. Some low-end printers are incapable of producing the same color across the page. Tints and solids may not print at the same density across the page. Hence a densitometer may measure very precisely an incorrect value due to the location of the sample, whereas an operator naturally compensates for such problems. PA1 Papers can be unreliable. Very often, lower quality papers are irregular in thickness and coating. Printed results can therefore vary widely, affecting densitometer measurements. PA1 An operator could feed the printer with a different paper than that expected by the calibration system. Also, an operator may not notice a certain spot on a specific sheet of paper where a sample is measured. This happens often because the operator has too much confidence in the densitometer and is less prone to verify his work. PA1 No calibration method is known that allows for visual calibration to arbitrary color response goals or different settings.